Polarity reversing circuit



Sept. 9, 1958 J. P. VINDING POLARITY REVERSING CIRCUIT Filed Aug. 1,1957 Mmur a /6 WW 3 w JNVENTOR.

Jaeaaw 2 l wpwc;

ITrOP/VFYJ United 2,851,543 Patented Sept. 9, 1958 art POLARITYREVERSENG CIRCUIT Jorgen 1P. Vinding, Monte Serena, Califi, assignor, bymesne assignments, to Monogram Precision Industries, Inc., Culver (Iity,Calih, a corporation of California Application August 1, 1957, SerialNo. 675,610

3 Claims. (Cl. 179-471) This invention relates to electrical circuitsfor reversing the polarity or phase of electric signals.

There is a frequent need for electrical circuits that will provide anoutput signal that is opposite in polarity or phase, but is otherwiseidentical, to an input electric signal. An ordinary grounded-cathodevacuum-tube amplifier stage provides an output signal of opposite phaseto the input signal, but such amplifier stages usually have alow-frequency cutoff below which they fail to operate properly, andtherefore they are not suitable for reversing the polarity of signalshaving substantial low-frequency and direct-current components. Certaindirect-coupled amplifiers have previously been devised thattheoretically have no low-frequency cutotf, and are operable down to thezero frequency (direct current), but such previous circuits commonlylack some or all of several desirable characteristics, including zerooutput voltage for zero input voltage, stability and freedom from drift,circuit simplicity and ruggedness, and operating characteristics thatare substantially independent of load resistance.

Briefly stated, according to certain of its aspects, this inventionprovides an improved polarity reversing circuit that combines desirableoperating characteristics with circuit simplicity and ruggedness. Theimproved circuit is a three-terminal active network having input, outputand common terminals. Responsive to an input voltage applied between theinput and common terminals, an output voltage of the opposite polarityis provided between the output and common terminals.

The circuit includes a cathode follower having anode connections to apositive voltage supply, and cathode connections to a negative voltagesupply, so that the cathode potential may vary through a range ofpositive and negative values relative to the potential of the commonterminal. The output terminal is connected to the cathode of the cathodefollower. A resistance voltage divider is connected between the inputterminal and an adjustable tap on a cathode resistor of the cathodefollower. The voltage divider is provided with a center tap. Aphase-inverting voltage amplifier is connected between this center tapand the control grid of the cathode follower, so that any smallpositive-going change in the potential of the center tap causes a muchlarger negativegoing change at the control grid of the cathode follower,and vice versa.

As a result, any positive-going change in the potential of the inputterminal (relative to the common terminal) causes an equalnegative-going change in the potential of the output terminal, and viceversa. The adjustable tap provides a calibrating means that can beadjusted to make the output voltage equal to zero when the input voltageequals zero. This calibrating adjustment is usually required only whenthe apparatus is initially set up, or when a vacuum tube is changed.Once the circuit is calibrated, it is exceptionally stable, issubstantially drift-free, and operates with low distortion from zerofrequency (direct current) through the audio frequency range.

The invention may be better understood from the following illustrativedescription and the accompanying drawing. The scope of the invention ispointed out in the appended claims.

In the drawing, the single figure is a simplified circuit diagram of animproved polarity reversing circuit.

Referring to thedrawing, the polarity reversing circuit illustratedincludes an input terminal 1, an output terminal 2, and a commonterminal or connection 3, which may be circuit ground. In its preferredform, the circuit includes two vacuum tube sections 4 and 5, which maybe the two halves of a twin triode vacuum tube, such as a type 12AT7tube. Each vacuum tube section has an anode, a control grid and acathode, as is indicated in the drawing by conventional symbols.

A positive voltage supply 6 is connected to supply a fixed positivepotential (+300 volts, for example) relative to common terminal 3. Anegative voltage supply "I, which preferably includes a voltageregulator tube 3 and dropping resistor 9, is connected to supply a fixednegative potential (-l50 volts, for example) relative to common terminal3. For example, regulator tube 8 may be a type 0A2 tube tube andresistor 9 may have a resistance of 4000 ohms, while the unregulatedvoltage across 7 is about -250 volts. The negative voltage supplypreferably is regulated with a voltage regulator tube, as shown, becausethe stability of the negative potential is critical to drift-freeoperation of the circuit. However, this is the only critical circuitvalue, and moderate variations in other circuit values, such as to beexpected with ordinary circuit components, have negligible effect uponthe circuit operation. Therefore, in contrast to most direct-coupledelectronic circuits, stability and substantially drift-free operation inthe present circuit are assured simply by regulating the value of onecriticalvoltage.

Vacuum tube section 4 is connected in a conventional phase-invertingdirect-coupled amplifier stage, and vacuum tube section 5 is connectedin a cathode follower stage. The anode of tube section 5 is connecteddirectly to positive voltage supply 6. The cathode of tube section 5 isconnected through a potentiometer 10 and a resistor 11, in series, tothe negative voltage supply, as shown. For example, the total resistanceof potentiometer 10 may be 1000 ohms, and the resistance of resistor 11may be 18,000 ohms. There is a direct circuit connection 12 between thecathode of tube section 5 and output terminal 2.

Potentiometer 10 has a conventional adjustable tap 13. Between thisadjustable tap and input terminal 1, there is connected a resistancevoltage divider consisting of two resistors 14 and 15 connected inseries, as shown. Preferably, the two resistors 14 and 15 have equalresistance values, 270,000 ohms each, for example. The circuit junction16 between the two resistors is the center tap of the voltage divider.

The anode of tube section 4 is connected to the positive voltage supplycircuit through a load resistor 17. The control grid of tube section 4-is connected to center tap 16 of the voltage divider. The cathode oftube section 4 is connected directly to common terminal 3, which may becircuit ground.

Another resistance voltage divider, consisting of two resistors 1d and19 in series, is connected between the anode of tube section l and thenegative voltage supply. The circuit junction between resistors 18 and19 is connected to the control grid of tube section 5. By way ofexample, resistor 17 may have a value of 47,000 ohms, resistor 18 mayhave a value of 560,000 ohms, and resistor 19 may have a value of680,000 ohms. This divider supplies to the control grid of tube sectionvoltage changes in phase with voltage changes at the anode of tubesection 4, while maintaining the control grid potential nearer groundpotential for proper opcration of the cathode follower stage. Thiscompletes the circuit.

The circuit may be calibrated as follows. Input terminals 1 and 3 areconnected directly together, by means of a jumper or the like, so thatthe input voltage between terminals 1 and 3 is held at zero. A voltmeteris connected to measure the output voltage between terminals 2 and 3.Now the position of tap 13 is adjusted. As tap 13 is moved upward, thecontrol grid of tube section 4 becomes more positive, and tube section 4conducts more current. This causes the anode of tube section 4 and thecontrol grid of tube section 5 to become more negative, whereupon tubesection 5 conducts less current. This causes the potential of outputterminal 2 to become more negative. Conversely, when tap 13 is moveddownward the potential of output terminal 2 becomes more positive.

A position of tap 13 can be found at which the measured voltage betweenterminals 2 and 3 is zero, and at this position of tap 13 the circuit isproperly calibrated. Because the circuit is quite stable, re-calibrationis usually required only when the vacuum tube is replaced. Aftercalibration has been completed, the jumper and voltmeter are removed,and the circuit is ready for operation.

It should be appreciated that the resistance between the cathode of tubesection 5 and adjustable tap 13 is only a small fraction of the totalresistance between cathn ode 5 and the negative voltage supply. Withzero input and output voltages, the total voltage drop acrosspotentiometer and resistor 11 in series is 150 volts, this being thevoltage that is provided by the regulated negative voltage supplyincluding the voltage regulator tube 8. The voltage drop between thecathode of tube section 5 and tap 13 must have twice the magnitude ofthe negative bias that must be supplied to tube section 4 to balance andcalibrate the circuit. Since the required negative bias is only a fewvolts, only a small fraction of the total voltage drop acrosspotentiometer 10 and resistor 11 is required. Therefore, the resistanceof resistor 11 preferably is considerably larger than the totalresistance at potentiometer 10, and, except for the bias and calibrationfunctions herein described, the resistance between the cathode of tubesection 5 and potentiometer 13 has little effect upon the circuitoperation.

Therefore, with respect to its dynamic operation, the circuit operatesessentially as though resistor 15 were connected directly to the cathodeof tube section 5. In fact, the small difference in dynamic operationdue to the resistance between the cathode of tube section 5 and tap 13acts to make the output voltage very slightly larger in magnitude thanit would be otherwise, and this tends to compensate for the finite gainof the amplifier stage (tube section 4), which tends to reduce theoutput voltage, so that differences in the magnitudes of the input andoutput voltages are kept quite small (in the order of one percent), andthe circuit operates with very low distortion over a range of inputvoltages between +12 volts and l2 volts.

Assume that there is applied between input terminal 1 and commonterminal 3, from any suitable source, an input voltage that variesbetween +12 volts and -12 volts. Whenever the instantaneous potential ofterminal 1 becomes more positive with respect to terminal 3, the controlgrid of tube section 4 becomes more positive. Since tube section 4 isconnected as a phase-inverting amplifier, the anode of tube section 4and the control grid of tube section 5 become more negative. Tubesection 5 operates as a cathode follower, and output terminal 2 and tap13 become more negative. This makes the potential of the control grid oftube section 4 more negative, and thus opposes the positive-going changecaused by an increase in the potential of terminal 1.

In actual operation, because of the considerable loop gain of thecircuit, changes in the potential of center tap 16 are relatively small,and any change in the potential of terminal 1 is substantially balancedby an almost equal change in the potential of tap 13. Because the loop.gain is finite, there must be some small change in the potential ofcenter tap 16 to operate the circuit, and the potential changes atadjustable tap 13 must be slightly smaller in magnitude than thepotential changes at input terminal 1. .T-lowever, the di .ercnce issmall, and even this small difierence tends to be compensated for by thevoltage drop across the upper part of potentiometer 10 between thecathode of tube section 5 and tap 13. This voltage drop causes thepotential change at output terminal 2 to be slightly larger (two orthree percent) than the potential changes at tap 13. Thus, the over-allaccuracy of the circuit is very good, and the potential changes ofoutput terminal 2 are almost exactly equal in magnitude (within one ortwo percent) but of opposite polarity to the potential changes at inputterminal 1.

It is thus apparent that the circuit illustrated and described operateswith exceptionally low distortion and provides an output signal betweenterminals 2 and 3 that is equal in magnitude, but opposite in polarityand phase, to an input signal provided between terminals 1 and 3. Thecircuit is operable from Zero frequency (direct current) through atleast the audio frequency range, and can be designed for operation ateven higher frequencies. Because the output is taken from the cathode ofthe cathode follower stage, the output impedance is low, and theoperating characteristics are substantially unaffected by reasonablevariations in the load impedance. There is only one simple calibrationadjustment (to set the zero level) and this calibration adjustment needordinarily be a made only when a tube is changed, since the circuit isexceptionally stable and drift-free. The only critical circuit value isthe stability of the negative voltage supply, which is easily assured bythe use of a good voltage regulator tube.

It should be understood that this invention in its broader aspects isnot limited to the specific example herein illustrated and described,and that the following claims are intended to cover all changes andmodifications within the true spirit and scope of the invention.

What is claimed is:

1. In an electrical circuit, the combination of an input terminal, anoutput terminal, a common terminal, a positive voltage supply connectedto provide a fixed positive potential relative to said common terminal,a negative voltage supply connected to provide a fixed negativepotential relative to said common terminal, a cathode follower havinganode circuit connections to said positive voltage supply, having acontrol electrode, and having cathode circuit connections to saidnegative voltage supply, a direct circuit connection between saidcathode circuit connections and said output terminal, a resistancevoltage divider connected between said cathode circuit connections andsaid input terminal, said voltage divider having a tap, and adirect-coupled, phase-inverting amplifier connected between said tap andsaid control grid.

2. A polarity reversing circuit comprising an input terminal, an outputterminal, a common terminal, a positive voltage supply connected toprovide a fixed positive potential relative to said common terminal, anegative voltage supply connected to provide a fixed negative potentialrelative to said common terminal, first and second vacuum tube sectionseach having an anode, a control grid and a cathode, a load resistorconnected between the anode of said first section and said positivevoltage supply, means connecting the anode of said second section tosaid positive voltage supply, means connecting the cathode of said firstsection to said common terminal, potentiometer means connected betweenthe cathode of said second section and said negative voltage supply,said potentiometer means having an adjustable tap, a first resistancevoltage divider connected between said adjustable tap and said inputterminal, said first voltage divider having a tap connected to thecontrol grid of said first vacuum tube section, a second resistancevoltage divider connected between the anode of said first vacuum tubesection and said negative voltage supply, said second voltage dividerhaving a tap connected to the control grid of said second vacuum tubesection, and a direct circuit 6 connection between the cathode of saidsecond vacuum tube section and said output terminal.

3. A polarity reversing circuit as defined in claim 2, wherein saidnegative voltage supply comprises a voltage regulator tube forstabilizing the value of said negative potential, whereby the polarityreversing circuit is made substantially drift-free.

No references cited.

